Along with a recent increase in the degree of integration of LSI's, these has been required a technique which permits display of accurate images, for instance, with minute lines of a width on the order of submicrons in a photolithography technique or a lithography technique for displaying an integrated circuit patter on a wafer. In the photolithography technique, an attempt has accordingly been done to adopt light rays of short wavelengths as light sources for exposure systems in order to form accurate images by lines having such a quite narrow width. There have been required, for instance, a stepper lense for lithography to have excellent transparency to ultraviolet rays, uniform refractive index distribution for ultraviolet rays, in particular, ultraviolet laser beams and high durability to the irradiation of ultraviolet laser beams in order to display, on a wafer, accurate and clear distortion-free images of integrated circuit patterns by fine lines without light and dark irregularities.
However, a lense made from a conventional optical glass currently used has very low transmittance to ultraviolet rays. For instance, when using ultraviolet rays having wavelengths falling within the wavelength range shorter than 365 nm (i-ray), the transmittance to ultraviolet rays is abruptly reduced during operating the optical system and it is thus substantially impossible to use such a lense as a stepper lense. In particular, when using ultraviolet rays having wavelengths falling within the wavelength range shorter than 365 nm (i-ray), the lense generates heat through absorption of the irradiated ultraviolet rays. This leads to changes of optical characteristics of the optical lense and in turn becomes a cause of a deviation of the focal length thereof. For this reason, quartz glass has been used as a material for producing parts through which ultraviolet rays are transmitted.
However, the quartz glass made of a naturally-occurring rock crystal has a low transmittance to ultraviolet rays having wavelengths of not more than 250 nm, further begins to absorb lights in the ultraviolet region during irradiation with ultraviolet rays and the ultraviolet transmission rate thereof is further reduced. It has thus been substantially impossible to use such quartz glass as a material for producing a stepper lense. It is believed that the quartz glass made of a naturally-occurring rock crystal absorbs light rays in the ultraviolet region due to impurities present in the quartz glass and for this reason, synthetic quartz glass having a low impurity-content, i.e., synthetic silica glass has been used for producing optical members which are used in the ultraviolet region.
In order to prevent any contamination with metallic impurities, the presence of which in general becomes a cause of the ultraviolet absorption, the synthetic quartz glass has been prepared by directly introducing, into an oxyhydrogen flame, vapor of a volatile, highly pure silicon compound which is chemically synthesized and purified through distillation, for instance, a silicon halides such as silicon tetrachloride (SiCl.sub.4); an alkoxysilane such as ethoxysilane (Si(OC.sub.2 H.sub.5).sub.4), methoxysilane (Si(OCH.sub.3).sub.4) or the like; or an alkylalkoxysilane such as methyltrimethoxysilane (SiCH.sub.3 (OCH.sub.3).sub.3), ethyltriethoxysilane (SiC.sub.2 H.sub.5 (OC.sub.2 H.sub.5).sub.3) or the like to cause flame-hydrolysis by the action of the oxyhydrogen flame, depositing and melting glass fine particles formed through the decomposition of the compound on a heat-resistant rod-like core material to give a transparent glass material. Alternatively, it is also possible to form a transparent glass material by depositing the foregoing glass fine particles on a heat-resistant rod-like core material to give a porous glass material and then heating and melting the porous glass material in an electric furnace.
The transparent synthetic quartz glass thus produced is quite pure, almost free of metallic impurities and can effectively transmit lights in a short wavelength region on the order of about 190 nm. Therefore, the synthetic quartz glass has been used as a material for transmitting lights from ultraviolet lasers, more specifically, excimer lasers such as KrF lasers (248 nm), XeCl lasers (308 nm), XeBr lasers (282 nm), XeF lasers (351, 353 nm) and ArF lasers (193 nm); and 4-fold higher harmonics (250 nm) of YAG lasers in addition to the foregoing i-line.
For instance, an attempt has been done to synthesize highly pure quartz glass having a content of elemental metallic impurities of not more than 0.1 ppm and comprising OH groups in a predetermined concentration by improving the purity of silicon tetrachloride as a starting material and controlling the conditions for flame-hydrolysis by an oxyhydrogen flame and to thus produce quartz glass parts for optical use having improved durability to lights from ultraviolet lasers (Japanese Unexamined Patent Publication No. Hei 1-167258).
Although the quartz glass parts for optical use prepared according to these methods exhibit excellent durability to lights from ultraviolet lasers, the production thereof requires an increase of the production processes. Therefore, they suffer from various problems from the viewpoint of production techniques and production time as well as on the economical standpoint.
Incidentally, the synthetic quartz glass likewise begins to absorb ultraviolet rays in a certain region upon irradiation with ultraviolet rays. This new absorption band within the ultraviolet region which is absorbed by the synthetic quartz glass would be exceptionally caused due to the presence of structures formed from components of the quartz glass other than SiO.sub.2 such as SiOH or SICl, or intrinsic defects due to oxygen excess- or deficient-structures such as Si--Si and Si--O--O--Si, which possibly generate paramagnetic defects through optical reactions. There have been detected and identified, by ESR spectrometry or the like, various paramagnetic defects in the synthetic quartz glass which become a cause of light absorption, for instance, E' center (Si.) and NBOHC (Si--O.).
As has been explained above, the paramagnetic defects in general have optical absorption bands. There have been known, for instance, an absorption at E' center, 215 nm, and at 260 nm (which has not yet been correctly identified) as examples of such absorption bands due to the paramagnetic defects in quartz glass falling within the ultraviolet region, which are observed upon irradiation thereof with ultraviolet rays. These absorption bands are relatively broad and strong and, therefore, this becomes a serious problem when it is used as a material for transmitting lasers such as ArF lasers (193 nm) and KrF lasers (248 nm).
For the foregoing reasons, the synthetic quartz glass used for excimer lasers must have high durability to ultraviolet rays such that it does not cause any new absorption band even when it is irradiated with strong ultraviolet rays such as light rays from ultraviolet lasers.
An object of the present invention is to provide an optical quartz glass optical members used in an optical system provided with an ultraviolet laser represented by an excimer laser as a light source, which permits the solution of the problem of the reduction in the ultraviolet transmission rate of the quartz glass optical member observed when it is irradiated with ultraviolet rays.